A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. For example, silane is commonly used as a source of silicon, and ammonia is used as a source of nitrogen.
CVD deposition is not restricted to the surface of the substrate, and this can result, for example, in the clogging of gas nozzles and the clouding of chamber windows. In addition, particulates may be formed, which can fall on the substrate and cause a defect in the deposited thin film, or interfere with the mechanical operation of the deposition system. As a result of this, the inside surface of the process chamber is regularly cleaned to remove the unwanted deposition material from the chamber. One method of cleaning the chamber is to supply a cleaning gas such as molecular fluorine (F2) to react with the unwanted deposition material.
Following the deposition or cleaning process conducted within the process chamber, there is typically a residual amount of the gas supplied to the process chamber contained in the gas exhaust from the process chamber. Process gases such as silane, ammonia and cleaning gases such as fluorine are highly dangerous if exhausted to the atmosphere, and so in view of this, before the exhaust gas is vented to the atmosphere, abatement apparatus is often provided to treat the exhaust gas to convert the more hazardous components of the exhaust gas into species that can be readily removed from the exhaust gas, for example by conventional scrubbing, and/or can be safely exhausted to the atmosphere.
One known type of abatement apparatus is described in EP-A-0 819 887. This abatement apparatus comprises a combustion chamber having an exhaust gas combustion nozzle for receiving the exhaust gas to be treated. An annular combustion nozzle is provided outside the exhaust gas nozzle, and a gas mixture of a fuel and air is supplied to the annular combustion nozzle for forming a reducing flame inside the combustion chamber for burning the exhaust gas received from the process chamber to destroy the harmful components of the exhaust gas.
In such an apparatus, the amount of fuel supplied to the combustion chamber is pre-set so that it is sufficient to destroy both the process and the cleaning gases contained within the exhaust gas. Due to the requirement to ensure a high destruction and removal efficiency (DRE) for fluorine-containing cleaning gases such as F2, NF3 and SF6, the total amount of fuel is typically determined by the calorific requirement to abate the maximum flow rate of cleaning gases that will enter the combustion chamber. CVD processes alternate between deposition and clean steps with a frequency that is determined by the tool type. Typically the process applications where the device described in EP-A-0 819 887 is used have a deposition step followed by a clean step. As a result, the abatement apparatus operates for around 50% of its time with a higher usage of fuel than is actually required to destroy the process gases associated with the deposition onto the substrate that is being processed.
Another problem that has been encountered with the use of a reducing flame is that a high DRE is not achieved when a high flow rate (for example, around 60 slpm) of exhaust gas containing ammonia is received, for example, from a flat panel display device process chamber.
It is an aim of at least the preferred embodiment of the present invention to seek to solve these and other problems.